Multi-layer thin film solar collector

ABSTRACT

A solar collector is provided that is formed from relatively thin polymer sheets. Three or more sheets are welded together to integrally define a storage tank, a plurality of layers of fluid flow channels (e.g. risers), and other desired features within the collector. The storage tank is located at a first end of the collector and a heat exchanger is provided within the storage tank. The heat exchanger is arranged to transfer heat from the storage tank to a target fluid (such as water) that is to be heated or preheated by the collector. The collector is arranged so that when it is installed, the collector may be positioned with the storage tank located generally above the fluid flow channels. The storage tank and the fluid flow channels are arranged such that when the collector is installed and filled with a working fluid, a thermosiphon effect will cause the collection of hot water within the storage tank.

BACKGROUND OF THE INVENTION

The present invention relates generally to solar heat collection panels. More particularly, the invention relates to thin film solar collectors.

Solar heat collection panels have been used for a number of years to heat or preheat water and/or other fluids for a number of applications. Many collectors utilize a metal collection plate having a number of metal riser tubes or tubes with fins that extend between header pipes (often called manifolds). Often, the collection plates are housed within an insulated box with a transparent glazing placed over the front surface of the collector panel.

Another type of solar heat collection panel contemplates eliminating the collection plate and rather simply runs a series of side-by-side riser tubes between a pair of header pipes. The header pipes and risers can be formed from a wide variety of materials, but one class of heat exchangers use simple extruded plastic or elastomer tubing. When plastic tubing is used as the risers, adjacent tubes can be extruded, tack welded or supported together so that an array of side-by-side tubes forms a collection panel. A variety of plastics may be used to form the panels, although generally a dark, thermoplastic material is used. Such panels have been sold for a number of years by FAFCO Inc. of Chico, Calif., and are described, for example, in U.S. Pat. No. 4,205,662.

Although these and other existing solar panel designs work quite well, there are continuing efforts to develop new collector designs that not only meet the needs of specific applications but also are also stronger and more reliable.

U.S. Pat. No. 4,143,644 describes a solar panel design that is formed by welding two plastic sheets together. Although the patent describes a low cost collector design, the design does not appear to have enjoyed substantial commercial success. It is believed that one of the reasons for this is that the described design appears to be highly susceptible to: (a) stress concentrations that can lead to collector failures at the transition between riser tubes and the bottom manifold; and (b) non-uniform thermosiphon fluid flow in the riser tubes since upward and replacement flow occur in the same layer.

SUMMARY OF THE INVENTION

In accordance with an embodiment, a solar collector is provided. The solar collector includes a front sheet formed from a polymer film, a middle sheet formed from a polymer film, and a back sheet formed from a polymer film. The front, middle, and back sheets are welded together, with the middle sheet between the front and back sheets, to define a storage tank, a manifold and two layers of fluid flow channels. Each layer of fluid flow channels includes a plurality of fluid flow channels that extend substantially longitudinally between the manifold and the storage tank. Each of the fluid flow channels opens directly into the storage tank, and the storage tank is located at a first end of the collector and the manifold is located at a second end of the collector.

In accordance with another embodiment, a solar collector is provided. The solar collector includes a front sheet formed from a polymer film, a middle sheet formed from a polymer film, and a back sheet formed from a polymer film. The front, middle, and back sheets are welded together, with the middle sheet being between the front and back sheets, to form a first layer of fluid flow channels between the front and middle sheets and a second layer of fluid flow channels between the middle and back sheets. Each layer of fluid flow channels includes a plurality of fluid flow channels that extend substantially longitudinally along the solar collector.

In accordance with yet another embodiment, a solar collector is provided. The solar collector includes a front sheet formed from a polymer film, a middle sheet formed from a polymer film, and a back sheet formed from a polymer film. The back sheet has substantially the same length as the front sheet and the middle sheet is shorter than the front and back sheets. The front, middle and back sheets are welded together to define first and second layers of substantially parallel fluid flow channels that extend longitudinally along the collector. The first layer is between the front sheet and the middle sheet and the second layer is between the middle sheet and the back sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a diagrammatic top view of a solar collector formed from welded thin films in accordance with an embodiment.

FIG. 2A is a cross-sectional perspective view of two layers of fluid channels of a solar collector in accordance with an embodiment.

FIG. 2B is a simplified side view of the embodiment of FIG. 1.

FIG. 2C is a simplified side view of an embodiment having a mesh strip at the bottom of the middle polymer sheet.

FIG. 2D is a perspective view of the risers of an embodiment having holes at the bottom of the middle polymer sheet.

FIG. 2E is a simplified side view of an embodiment having a heat exchanger in a third layer containing a flow path.

FIG. 2F is a simplified side view of an embodiment having a heat exchanger in a third layer containing a flow path and without a storage tank.

FIG. 3 is a diagrammatic illustration of a solar collection system incorporating a solar collector.

FIG. 4 is a perspective view of a filled solar collector in accordance with an embodiment.

FIG. 5 is a diagrammatic perspective view of a foam insulation suitable for the storage tank portion of the collector illustrated in FIG. 1.

FIG. 6A is a perspective view of an embodiment of a heat exchanger unit.

FIG. 6B is a cut-away view of a storage tank containing another embodiment of a heat exchanger unit.

FIG. 7 is a perspective view of an exemplary heat exchanger port that is suitable for the collector.

FIG. 8A is a perspective view of an exemplary fill and drain port in accordance with an embodiment.

FIG. 8B is a perspective view of an exemplary fill and drain port in accordance with another embodiment.

FIG. 9 is a diagrammatic plan view of welded thin films for a collector without a storage tank in accordance with another embodiment of the invention.

FIG. 10 is a cross-sectional perspective view of two layers of fluid channels of a solar collector with a glazing layer welded to the front side and a foam insulation on the back side in accordance with an embodiment.

FIG. 11 is a cross-sectional perspective view of two layers of fluid channels of a solar collector with a foam insulation on the back side in accordance with an embodiment.

It is to be understood that, in the drawings, like reference numerals designate like structural elements. Also, it is understood that the depictions in the figures are diagrammatic and not to scale.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates generally to solar heat collection panels formed from relatively thin film sheets.

Referring initially to FIG. 1, a solar collection system in accordance with one embodiment of the invention will be described. In the illustrated embodiment, a solar collector 20 is formed from three sheets of a thin film polymer, such as black polyethylene or polypropylene. The polymer sheets are welded together to form two layers of risers, using conventional plastic welding techniques such as impulse welding (heat sealing) or platen welding. It will be understood that, in other embodiments, four or more thin film polymer sheets can be welded together to form a solar collector having three or more layers of risers.

In the context of this description, the terms “top”, “bottom”, “front”, “back” and “vertical” are used primarily in the context of describing the drawings. It should be appreciated that when the solar collectors are installed, the storage tank would typically be positioned above the riser portions of the collector, although the collector would typically not be mounted vertically. Rather, it would be installed at an angle that is appropriate for the location at which the collectors are installed.

In the illustrated embodiment, the welds are arranged to define a closed collector having an integrally formed storage tank 23 and two layers, each having a multiplicity of adjacent riser type fluid channels 26. FIG. 2 is a cross-sectional perspective view of the two layers of fluid channels 26. As shown in FIG. 1, the welds include a series of peripheral collector body welds 29 that define the height and width of the collector 20 when it is filled. The welds also include a plurality of substantially parallel riser welds 32, 33 that define two layers of fluid channels or risers 26 that make up the bulk of the collector portion of the system.

As shown in the embodiment illustrated in FIGS. 1 and 2, a front sheet 60 of a thin film polymer is welded to a middle sheet 62 of a thin film polymer, and a back sheet 64 of a thin film polymer is also welded to the middle sheet 62. A welding apparatus can be set to weld the front sheet 60 only to the middle sheet 62 with the three sheets stacked together (without welding the middle sheet 62 to the back sheet 64). Similarly, the welding apparatus can be set to weld the back sheet 64 only to the middle sheet 62 with the three sheets stacked together (without welding the middle sheet 62 to the front sheet 60).

As shown in FIG. 2A, the front sheet 60 is welded to the middle sheet 62 and the back sheet 64 is welded to the middle sheet 62 in an alternating manner. In other words, moving laterally across the middle sheet 62, the welds 32 on the middle sheet 62 alternate between a weld 32 to the front sheet 60 and a weld 33 to the back sheet 64. As shown in FIG. 1, the welds, as illustrated, alternate between a solid line 32 and a dashed line 33. The solid lines represent welds 32 of the front sheet 60 to the middle sheet 62 whereas the dashed lines 33 represent welds of the middle sheet 62 to the back sheet 64. Alternatively, the solid lines can represent welds of the middle sheet 62 to the back sheet 64 whereas the dashed lines represent welds of the middle sheet 62 to the front sheet 60. Although, some of the welds 33 are represented with dashed lines in FIG. 1, it will be understood that all of the welds 32, 33 of the collector 20 are continuous unless otherwise indicated. It will be understood that, in FIG. 1, dashed lines are used solely for the purpose of illustrating where the different welds are located and that welds 32 and 33 alternate across the collector.

As shown in FIG. 2A, when filled with working fluid, the fluid channels 26 have substantially the cross-sectional shape as illustrated. As illustrated in FIG. 2A, each fluid channel 26 does not have a round cross-section, but rather a curved, substantially triangular cross-section.

In the illustrated embodiment, with three polymer sheets 60, 62, 64, the solar collector 20 has two layers 70, 72 of risers or fluid channels 26. There is a front layer 70 of risers 26 between the front sheet 60 and the middle sheet 62, and there is a back layer 72 of risers 26 between the back sheet 64 and the middle sheet 62. As shown in FIG. 2A, the middle sheet 62 forms the “bottom” surfaces of the risers 26 of the front layer 70, and the middle sheet 62 forms the “top” surfaces of the risers of the back layer 72. It will be understood that if the solar collector 20 has additional polymer sheets welded to the front or back sheet, the collector 20 will also have additional layers of risers.

In the embodiment illustrated in FIG. 1, the storage tank 23 is integrally formed with the risers 26 so that the risers 26 open directly into the storage tank 23. With this arrangement, thermosiphonic flow between the storage tank 23 and the main body of the collector panel is not limited to a single narrow channel, which both simplifies the design and reduces the probability of kink or other obstructions severely hampering the efficacy of the collector.

In the context of the collector 20, a storage tank 23 is positioned on the “top” side of the collector 20 and the riser welds 32 extend “vertically” downward below the storage tank 23. In order to form the integral storage tank 23, the middle sheet 62 is shorter than the front and back sheets 60, 64.

FIG. 2B is a simplified side view of the embodiment of FIG. 1. It will be understood that, for simplicity, the accordion-like nature of the sheets (which is shown in FIG. 2A) is not illustrated in the side view of FIG. 2B. As shown in FIG. 2B, the middle sheet 62 is shorter than the front and back sheets 60, 64. The top edge of the middle sheet 62 is below the storage tank 23 where the riser welds 32 start extending “vertically” below the storage tank 23. In the embodiment illustrated in FIGS. 1 and 2, the middle sheet 62 does not extend all the way to the top peripheral weld 29 so that a storage tank 23 can be formed at the top of the collector 20 between the front and back sheets 60, 64.

Also, in this embodiment at the bottom of the collector 20, the middle sheet 62 does not extend all the way to the bottom peripheral weld 29. With this arrangement, a manifold 38 is effectively formed at the bottom end of the collector body so that a working fluid within the collector body can flow between hot and cold paths within a riser 26, as shown in FIGS. 1 and 2B, and also allows filling and draining of the collector fluid channels.

FIG. 2B also illustrates exemplary hot and cold flow paths of the working fluid. As will be explained in more detail below, the collector 20 functions as a thermosiphonic system. In this embodiment, a hot thermosiphonic flow path is provided through the front layer 70 of risers 26 and a downward cold flow path is provided through the back layer 72 of risers 26.

Alternatively, in another embodiment, the middle sheet 62 can extend all the way to the bottom peripheral weld 29 and holes and/or a mesh strip can be provided in the middle sheet 62 near the bottom to allow flow of the working fluid between layers 70, 72. The riser welds 32 extend all of the way to the peripheral collector body weld 29 at the “bottom” of the collector 20 so that adjacent risers 26 in the same layer are separated from one another. As shown in FIG. 2C, in one embodiment, a mesh strip 76 is provided at the bottom of the middle sheet 62 to allow flow between the front and back layers 70, 72. According to another embodiment shown in FIG. 2D, instead of a mesh strip, holes 68 are provided in the middle sheet 62 in the area of the mesh strip 76 of the embodiment shown in FIG. 2C. In this embodiment, the holes 68 allow flow between the front and back layers 70, 72. By allowing flow between the layers 70, 72 for filling and draining of the system, the holes 68 and/or mesh strip 76 in the middle layer 62 also allow the riser welds 32 to extend all the way to the bottom peripheral weld 29 to reduce stress points. A potential problem with a conventional solar collector having only a single layer of risers is that there is some risk that some of the welds will fail, especially where stresses tend to congregate at the ends of the riser tube welds 32 which forms the manifold. However, in the described embodiments, there is less stress in these areas because the welds 32 reach all the way to the peripheral weld 29. Reinforcement is therefore unnecessary and the collector can be operated at higher pressures without risk of delamination of the sheets.

As discussed above, in the embodiment shown in FIG. 1, the riser welds 32 and middle layer 62 can end short at the bottom to allow fluid to flow freely between adjacent risers 26 via the manifold 38. In such an embodiment, holes or mesh in the middle layer 62 are unnecessary. The bottom weld 29 (see FIG. 1) relieves much of the stress that would concentrate at the channel weld ends in the two layer configuration.

The solar collector 20 also has at least one fill and drain port 35 that permits the collector 20 to be filled with (and drained of) a suitable working fluid, such as water or an antifreeze solution (e.g., brine or a glycol type solution). In the embodiment illustrated in FIGS. 1 and 2B, the fill and drain port 35 is located at the bottom of the collector body so that it opens into the manifold 38. In the embodiments shown in FIGS. 2C and 2D, the fill and drain port can be located near the bottom of the collector body so that it opens into the area of the mesh strip or holes in the middle sheet 62. However, in alternative embodiments, the fill port may be located at any other suitable location, as for example, above the storage tank 23 or along one of the sides of the collector 20 such that it opens into the collector body. Typically, only one fill and drain port 35 is required since the collector body is defined in a way such that all of the risers 26 open into the storage tank 23 at one end and, in some embodiments, into a manifold 38 at their other end. Accordingly, the entire collector 20 can be filled or drained through a single fill and drain port 35. However, in alternative embodiments, multiple fill and drain ports may be provided. As shown in FIG. 1, an air port 44 may be provided to facilitate venting air from the collector when it is being filled. In some embodiments, the air port 44 may be eliminated.

In the embodiments shown, the fill and drain port 35 and the air port 44 are formed by simply defining a fill channel and an air channel in the collector body that is open to the outside of the collector body. The collector 20 can then be filled by inserting a hose, a funnel, or other suitable fluid supply source into the fill channel Any air that gets trapped within the collector during filling may be released through the air port 44 before, during or after the collector is filled. After the collector has been filled, the air and fill ports may be closed by a valve, or by folding over and clipping a tube to form a water-tight seal. Alternatively, an air bleed valve can be used.

In still other embodiments, a plastic fitting or other appropriate connector may be welded to, clamped or otherwise attached to the fill port (e.g., using a hose clamp). A removable (e.g. threaded) cap, valve or plug can then be secured to the fitting to seal the fill port. Embodiments of suitable fill and drain ports are described below with reference to FIGS. 8A and 8B.

In the illustrated embodiment, as shown in FIG. 1, the solar collector 20 also has a pair of tie down channels 41, 42 that are arranged to receive external bars, rods, straps, or the like that can be used to secure the solar collector 20 in place when it is installed. The tie down channels 41, 42 are open on both ends and are located outside of the main collector body so that they are sealed from the collector body by at least the peripheral collector body welds 29. Upper tie down channel 41 is provided above the storage tank 23 and lower tie down channel 42 is provided below the manifold 38 in the illustrated embodiment. With this arrangement, bars, rods, straps or other suitable supporting structures can be inserted through the tie down channels 41, 42 and secured to a supporting fixture or other structure that the solar collector 20 is mounted on in order to hold the solar collector 20 in place Like the other integral components of the solar collector 20, the tie down channels 41, 42 are defined by welding the sheets 60, 62, 64 together at appropriate locations. It should be appreciated that the placement, size and configuration of the tie down channels may be widely varied to meet the needs or any particular application. In the illustrated embodiments, the tie down channels may be located in other appropriate places, as for example, along the sides of the collector.

In some embodiments, the tie down channels may extend the entire width of the collector panel. This type of arrangement may be particularly appropriate in situations where no ports are provided along the edge with which the tie down channel is aligned. In other instances, the tie down channels may extend only a portion of the width of the collector panel, or may be segmented. In the embodiment shown in FIG. 1, the tie down channel 41, 42 are continuous and extend the entire width of the panel.

In some embodiments, the heat exchanger can be positioned in the storage tank 23. In the embodiment shown in FIG. 1, the collector has a pair of heat exchanger ports 52 located in the storage tank. In the embodiment illustrated in FIG. 1, the heat exchanger ports 52 are illustrated on the front side of the storage tank 23. However, it will be understood that in other embodiments and depending on the type of heat exchanger provided, the heat exchanger ports 52 may me positioned on the back side of the storage tank 23 or one port on each side of the storage tank 23. Further, it may be desirable to provide only a single heat exchanger port. As will be described in more detail below, the heat exchanger ports 52 are arranged so that a heat exchanger can be inserted into the storage tank and plumbed as desired to integrate the collector into a solar heat collection system. Alternatively, a plastic manifold may be welded into the storage tank 23 to form the heat exchanger port 52.

In another embodiment shown in FIG. 2E, the heat exchanger can be formed by a third layer 76. The third layer 76 is formed by a fourth sheet of thin film polymer welded to one of the outer sheets, either the front sheet 60 or the back sheet 64. As shown in FIG. 2E, because of the weight of the storage tank 23, spacers 78 can be used to support the flow path beneath the storage tank 23 to maintain efficiency of the collector. Porous spacer 78 can be used and can take the shape of a helical coil or can be a plastic mesh. In the illustrated embodiment, the fourth sheet 66 is welded to the back sheet 64. FIG. 2F is a side view of a collector having a third layer 76 acting as a heat exchanger, but the collector does not include a storage tank.

Referring next to FIG. 3, one representative solar collection system 100 incorporating the described solar collector will be described. In the illustrated system 100, a collector 20 is utilized as a pre-heater for a conventional hot water tank or water heater 103. The collector 20 can be mounted on a suitable support structure (not shown). The support structure may be free standing, mounted on a roof, mounted on a wall or in any other suitable arrangement or the collectors may simply be placed on the ground. In the illustrated embodiment, the collector 20 is plumbed so that water from a water main (or other suitable water supply) enters the heat exchanger module 50 for the solar collector 20 and is routed to the hot water heater 103. It will be appreciated that in other embodiments, different plumbing arrangements may be used.

It should be appreciated that although a particular water preheating system is described, the described collectors may be used in a very wide variety of different systems. For example, they may be used as the sole source of hot water, in pool heating applications, as water heaters for radiant space heating applications, etc. Of course, any suitable number of solar collectors 20 may be used within the solar collection system. In some arrangements, it may be desirable to plumb multiple collectors together in series, whereas in other applications it may be desirable to plumb multiple collectors together in parallel or in other arrangements.

It should be apparent that the described collectors do not have any internal pumps or other moving parts that are arranged to circulate the fluid within the collector. Thus, the working fluid within the collector is not pressurized (although it will typically be desirable to pressurize the external water that passes through the heat exchanger). Thus, the collector works as a thermosiphon system.

With three or more layers of thin film sheets, flow paths can be separated. For example, a front layer 70 of channels or risers 26 is provided for hot water thermosiphon flow and a back layer 72 of channels or risers 26, which is separated from the front layer 70, is provided for downward flow of cold water, as shown in FIGS. 2B-2F.

Since the collectors 20 are made from thin film materials, the collector 20 will effectively inflate when filled with a working fluid (e.g., water or an antifreeze solution). FIG. 4 is a perspective view of a filled solar collector in accordance with an embodiment. As seen therein, from outside, the collector has an appearance that is somewhat similar to an air mattress. The dimensions of the collector, the size of the storage tank, and the widths/diameters of the risers may be widely varied. By way of example, a prototype of the embodiment illustrated in FIG. 4 can be formed from 4 foot by 12 foot sheets of plastic. When the collector is filled with fluid, the width decreases by a factor of close to 2/πr. Each collector in this embodiment holds approximately four gallons of water with the storage tank holding approximately 25 gallons. Of course, the overall size of the collector, the sizes of the risers and the size of the integral storage tank may all be widely varied to meet the needs of a particular application.

Generally, any size sheets may be used. By way of example, widths on the order of 1½ to 10 feet (more preferably about 4 feet) and heights on the order of 4-14 feet (more preferably 12 feet) are believed to be appropriate for most applications. The widths of the risers may also be widely varied. By way of example, unfilled riser widths of ¼ to 6 inches (more preferably ½ to 1½ inches) are believed to be appropriate for most applications. The dimensions of the storage tank can also be widely varied. By way of example, in the embodiments shown in FIG. 4, the height of the storage tank is approximately 16 inches. Generally, storage tank heights on the order of 6 to 24 inches are believed to be appropriate for most applications. With these arrangements, the thickness of the filled storage tanks tend to be on the order of 3 to 10 inches. Although some dimensions are described that are believed appropriate for most applications, it should be appreciated that these dimensions may be widely varied to meet the needs of a particular application.

The described collectors are extremely inexpensive to produce. The collection efficiency of the described collectors is less than most conventional solar collector designs. However, they have a calculated solar fraction (SF) of about 50-65% in sun belt locations for a collector area of about 60 square feet. The design philosophy was to produce solar heat at relatively low efficiencies, but at a very low installed cost while minimizing stagnation and freezing risks.

As discussed above, the described arrangement is a passive collector that has a thermosiphonic flow. Common thermosiphon designs have a designated flow channel for carrying fluid to the tank, and a separate one for carrying fluid to the bottom collector manifold. Experiments have demonstrated that with the described arrangement, at any given time, the risers 26 in one layer 70 carry the working fluid upward, while the risers 26 in the other layer 72 carry the working fluid downward, as shown in FIGS. 2B-2F.

The collectors can be unglazed, as illustrated in FIGS. 1-2. It is also possible to leave the storage tank uninsulated. However, in many applications it may be desirable to insulate the storage tank portion of the collector. The insulation can be accomplished using a variety of techniques. A rigid foam insulation 111 suitable for insulating storage tank 23 portion of the collector 20 is shown in FIG. 5. In another embodiment, the insulation can be formed on the storage tank 23 by multiple layers of ¼ or ½ inch flexible foam (e.g., polyethylene) folded over the storage tank 23 from the top, extending under the tank 23 to the collector, and over the top of the tank to the collector. The cover can also be folded over from the top, with the edges at the collector supported by rods, and the sides fastened together.

The heat exchanger 50 may take a wide variety of forms. Preferably, the heat exchanger is a self-contained unit that may be sealed inside the storage tank and plumbed to the heat exchanger ports 52. Representative heat exchanger units 50 are illustrated in FIGS. 6A-6B. FIG. 6B is a cut-away view of a storage tank 23 with an embodiment of a heat exchanger unit 50 therein. The heat exchanger ports 52 may also be formed in a wide variety of manners.

A polymer heat exchanger 50 can be used with the collector 20. The heat exchanger 50 includes a multiplicity of plastic tubes having a pair of manifolds on opposing ends. In an embodiment, a threaded portion of the manifold can be screwed into the heat exchanger port 52 to form an appropriate seal on both ends of the heat exchanger 50.

The diameter and length of the tubes may be widely varied to meet the needs of any particular application. Preferably, the tubes will have small diameters and relatively thin walls. By way of example, polymer tubes having an outer diameter in the range of approximately 0.08 to 0.25 inches work well for many applications, although both larger and smaller tube diameters may be used in particular applications. Inner tube diameters may also vary widely although it should be appreciated that thin walls are generally preferred since thin walls will generally improve the heat exchangers thermal performance by decreasing the temperature drop across the tube walls. In a preferred configuration the tubes are separated from each other and bent into a serpentine configuration.

The manifolds may take a variety of geometries and are preferably configured to facilitate coupling to fluid supply and drain conduits. FIG. 7 is a perspective view of an exemplary heat exchanger port that is suitable for the collector. For simplicity, FIG. 7 does not show threads into which the threaded portion of the manifold 104 may be screwed.

The tubes and manifolds may be formed from a wide variety of plastics and other polymers. By way of example, Polyethylene, Polypropylene, Polyamide, Polysulfone, and Polyphenylene Sulfide work well for both the tubes and the manifolds. Of course, in other embodiments, a wide variety of other plastics and polymers may be used. The tubes and manifolds may be formed from the same materials, substantially the same materials or different materials depending on the requirements of any particular application.

Of course, in alternative embodiments, a variety of other heat exchanger designs can be used. In some embodiments, the heat exchanger may be provided with end caps that mate with the heat exchanger ports 52. The end cap design would vary based on the design of the corresponding heat exchanger port. The fill and drain port 35 may also be configured in a variety of manners. FIG. 8A shows an embodiment of a fill and drain port 35 that is suitable for the collector. FIG. 8B is a cut-away view of another embodiment of a suitable fill and drain port 36.

In the initially described embodiment, the collector 20 is formed by welding three sheets of plastic together in a manner that defines the storage tank 23, two layers of side-by-side risers 26, ports and other components. It should be appreciated that in practice, the three sheets that are welded together may be formed from three or more separate rolls of film material. Those familiar with thin film sheet production will know that films of this type are sometimes formed in a tubular manner and an extended length of the film is provided on a roll.

A variety of different materials can be used to form the collector. Preferably, the material is one that resists degradation and otherwise holds up well under direct exposure to sunlight. By way of example, 6 mil thick black polypropylene or polyethylene work well. Of course the thickness of the material can be widely varied.

One feature of the described collector arrangement is that the outer layers can be thicker than the middle film layer, which will pillow out less when filled.

One of the significant advantages of the described thin film collector is that the geometry of the collector, the flowpaths within the collector, the size, number and geometry of the storage tanks, risers, manifolds, ports and the other components may be widely varied to meet the needs of a particular application with minimal effort (i.e., simply by setting up the welds appropriately for a desired new design).

Referring next to FIG. 9, another embodiment will be described. In this embodiment, the storage tank and the heat exchanger are eliminated so that the collector 400 directly heats the water (or other fluid) to be heated. The collector 400 has a plurality of risers 26. The welds 432 of the risers 26 of the top and bottom sheets extend all the way from the top peripheral weld 429 to the bottom peripheral weld 429. In this embodiment, the middle sheet also extends all the way from the top peripheral weld 429 to the bottom peripheral weld 429 and holes and/or a mesh strip can be provided in the middle sheet near the bottom to allow flow of the working fluid between layers, or the middle sheet can be shorter. The flow between layers takes place in the areas 438 just below the top peripheral weld 429 and in the area 439 just above the bottom peripheral weld 429. As noted above, the riser welds 432 extend all of the way from the top peripheral weld 429 to the bottom peripheral weld 429 so that adjacent risers 26 in the same layer are separated from one another. The “holes and/or mesh strip” in the middle sheet are similar to those shown in FIGS. 2C and 2D. The holes and/or mesh strip allow flow between the front and back layers. By allowing flow between the layers for filling and draining of the system, the holes and/or mesh strip in the middle layer also allow the riser welds 432 to extend all the way from the top peripheral weld to the bottom peripheral weld 29 to reduce stress points. A potential problem with a conventional solar collector having only a single layer of risers is that there is some risk that some of the welds will fail, especially where stresses tend to congregate at the ends of the riser welds 32. However, in the described embodiments, there is less stress in these areas because the welds 32 reach all the way to the peripheral weld 29 and because the multi-layer configuration provides more rigidity. Reinforcement is therefore unnecessary and the risk of delamination of the sheets is reduced with the additional layers.

The water enters the collector through an inlet 442 and exits through an outlet 444. The position of the inlet and outlet may be widely varied, although generally it is preferable to have the inlet at or near the bottom and the outlet at the top of the collector.

It is well understood in the thermal solar collection arts that glazing can be used to improve the thermal efficiency of a collector. Thus, in an embodiment, a thin film glazing layer 61 can be provided on a solar collector, as shown in FIG. 10. A transparent (or translucent) thin film glazing layer 61 can be welded to the front collector sheet 60 so that an enclosed space is formed between the front sheet 60 of the collector and the glazing 61. An appropriate air fill port can be provided between the glazing layer and the front sheet. Intermediate portions of the glazing 61 can be attached to appropriate intermediate portions of the upper collector sheet (e.g., the middle of the risers). With this arrangement, air can be pumped into the region between the glazing layer 61 and the front collector sheet 60. The air and glazing act as an insulator that insulates the exposed surface of the collector. With this arrangement, a very low cost glazing can be integrally formed with the collector. The glazing may be attached to every riser or a smaller set of the risers to thereby form larger air pocket channels (e.g., that extend over 2, 3, 4 or more adjacent risers as is appropriate for a particular design). In other embodiments, other intermediate glazing weld geometries can be used.

With the described arrangement, the stagnation temperature of the collector can be controlled somewhat by adjusting the amount of air (which acts as an insulator) within the air pockets. If the glazing is completely deflated, then the stagnation temperature of the collector under any given operating condition will be reduced compared to a state in which the air pocket are fully inflated. This feature makes it possible to control the stagnation temperature of the collector somewhat by controlling the level to which the glazing is inflated.

It should be appreciated that the glazing effectively insulates the top surface of the collector. In alternative embodiments, a similar air gap type insulation layer can be provided over other surfaces of the collector. By way of example, a thin film insulating layer similar to the glazing layer can be attached to the bottom surface of the back collector sheet 64 in a similar manner. When the region between the insulating layer and the back collector sheet is filled with air, it serves as an insulator for the bottom surface of the collector. Of course, if the insulating layer is not itself exposed to sunlight, there is no need for it to be transparent or translucent. Similarly, air gap type insulating layers can be provided over any or all surfaces of the storage tank, or polymer foam 65 can be welded to the back collector sheet 64, as shown in FIGS. 10 and 11. FIG. 11 shows the risers without a glazing layer on the front side but with the polymer foam 65 on the back collector sheet 64.

The described thin film collectors are very versatile and can be used in a wide variety of applications. In the primary described embodiments, the collector is a passive device that does not have any pumps or the like. The water (or other working fluid) within the collector circulates due to thermosiphonic flow. The heat exchanger receives pressurized water (as for example city water), which is heated within the storage tank and fed to the desired location (e.g., to a hot water heater, to an appropriate tap, etc.).

Thin film collectors can also be used in active systems, with or without a heat exchanger, in which some mechanism would be needed to circulate the water to be heated. For example, a pump could circulate fluid through the internal heat exchanger in the film collector to heat fluid in an external storage tank. Alternatively, if the collector is used without an internal heat exchanger, water may be pumped through the collector and then fed directly into the system in which it will be used. In one example, if the water entering the collector is pressurized (e.g. city water) a float value could be used to control the delivery of water into the collector.

In the primary described embodiments, water or an antifreeze solution is used as the working fluid. However, it should be appreciated that a wide variety of other working fluids could be used. By way of example, air could be used as the working fluid if the collectors are used in a space heating application. Such a collector would typically be configured without a storage tank or heat exchanger.

Although only a few embodiments of the invention have been described in detail, it should be appreciated that the invention may be implemented in many other forms without departing from the spirit or scope of the invention. Indeed, it should be appreciated that one of the design strengths of the described thin film collector is that the geometry of the collector, the flow paths within the collector, the size, number and geometry (and in some cases existence) of the storage tanks, risers, manifolds, ports and the other components may be widely varied to meet the needs of a particular application with minimal effort.

It should be apparent that the described thin film collectors can be used in a wide variety of applications. The peripheral welding provides a good seal and the various ports can readily be sealed so that evaporation of the working fluid is minimized during use.

In view of all of the foregoing, it should be apparent that the present embodiments are illustrative and not restrictive and the invention is not limited to the details given herein, but may be modified within the scope and equivalents of the appended claims. 

1. A solar collector, comprising: a front sheet formed from a polymer film; a middle sheet formed from a polymer film; and a back sheet formed from a polymer film, wherein the front, middle, and back sheets are welded together, with the middle sheet being between the front and back sheets, to define a storage tank, a manifold and a front layer of fluid flow channels and a back layer of fluid flow channels, wherein a plurality of longitudinal welds define the fluid flow channels, wherein some of the longitudinal welds are only between the front sheet and the middle sheet to form the front layer of channels and other of the longitudinal welds are only between the middle sheet and the back sheet to form the back layer of channels, wherein each layer of fluid flow channels comprises a plurality of fluid flow channels that extend substantially longitudinally between the manifold and the storage tank, wherein each of the fluid flow channels opens directly into the storage tank, the storage tank being located at a first end of the collector and the manifold being located at a second end of the collector, and wherein the front layer of fluid flow channels provides a hot thermosiphonic flow path therethrough in a first direction and the back layer of fluid flow channels provides a cold flow path therethrough in a second direction, wherein the first and second directions are opposite directions.
 2. The solar collector as recited in claim 1, wherein the middle sheet is shorter than the front and back sheets such that the storage tank is formed between the front and back sheets.
 3. The solar collector as recited in claim 1, wherein the manifold allows fluid flow between the two layers of fluid flow channels.
 4. The solar collector as recited in claim 3, wherein the manifold is formed by a mesh strip attached to a bottom end of the middle sheet.
 5. The solar collector as recited in claim 3, wherein the manifold is formed by providing a plurality of holes at a bottom end of the middle sheet.
 6. The solar collector as recited in claim 1, wherein the front and back sheets are further welded to define elongated tie down channels on top and bottom ends of the solar collector, the tie down channels being arranged to receive a tie down mechanism and not being in fluid communication with either the storage tank or the fluid flow channels, wherein the elongated tie down channels each have a longitudinal axis that extends substantially perpendicular to the fluid flow channels in a plane defined the fluid flow channels.
 7. The solar collector as recited in claim 1, further comprising a heat exchanger disposed in the storage tank, the heat exchanger having an inlet and an outlet at the storage tank.
 8. A solar collector, comprising: a front sheet formed from a polymer film; a middle sheet formed from a polymer film; and a back sheet formed from a polymer film, wherein the front, middle, and back sheets are welded together by a plurality of longitudinal welds, with the middle sheet being between the front and back sheets, wherein some of the longitudinal welds are only between the front sheet and the middle sheet to form the front layer of channels between the front and middle sheets and other of the longitudinal welds are only between the middle sheet and the back sheet to form the back layer of channels between the middle and back sheets, wherein each layer of fluid flow channels comprises a plurality of fluid flow channels that extend substantially longitudinally along the solar collector, and wherein the first layer of fluid flow channels provides a flow path therethrough in a first direction and the second layer of fluid flow channels provides a flow path therethrough in a second direction, wherein the first and second directions are opposite directions.
 9. The solar collector as recited in claim 8, further comprising a manifold at a bottom end of the collector, the manifold allowing fluid flow between the first layer and second layer of fluid flow channels.
 10. The solar collector as recited in claim 9, further comprising a storage tank at a top end of the collector, wherein the fluid flow channels extend longitudinally between the manifold and the storage tank, wherein each of the fluid flow channels opens directly into the storage tank.
 11. The solar collector as recited in claim 10, further comprising a heat exchanger disposed in the storage tank, the heat exchanger having an inlet and an outlet at the storage tank.
 12. The solar collector as recited in claim 10, wherein the middle sheet is shorter than the front and back sheets to define the storage bank at the top end of the collector, the storage tank being formed between the front and back sheets.
 13. The solar collector as recited in claim 8, further comprising a fill port that may be used to facilitate filling the fluid flow channels and the storage tank portions of the solar collector with a working fluid.
 14. A solar collector system comprising: a front sheet formed from a polymer film; a middle sheet formed from a polymer film; and a back sheet formed from a polymer film, the back sheet having a substantially same length as the front sheet and the middle sheet being shorter than the front and back sheets wherein the front, middle and back sheets are welded together by a plurality of longitudinal welds to define first and second layers of substantially parallel fluid flow channels that extend longitudinally along the collector, the longitudinal welds on the middle sheet alternate between a weld to the front sheet and a weld to the back sheet, wherein the first layer is between the front sheet and the middle sheet and the second layer is between the middle sheet and the back sheet, and wherein the first layer of fluid flow channels provides a hot thermosiphonic flow path therethrough in a first direction and the second layer of fluid flow channels provides a cold flow path therethrough in a second direction, wherein the first and second directions are opposite directions.
 15. The solar collector as recited in claim 14, further comprising a storage tank at a first end of the collector, the fluid flow channels being in fluid communication with the tank and arranged to open into the tank.
 16. The solar collector as recited in claim 15, wherein the storage tank is between the front sheet and the back sheet at the first end of the collector to which the middle sheet does not extend.
 17. The solar collector as recited in claim 14, further comprising a manifold at a second end of the collector, the manifold allowing fluid flow between the first and second layers.
 18. The solar collector as recited in claim 14, wherein the middle sheet extends neither to the top nor bottom end of the collector.
 19. The solar collector as recited in claim 14, further comprising a heat exchanger disposed in the storage tank, the heat exchanger having inlet and outlet ports.
 20. The solar collector as recited in claim 14, further comprising a glazing layer over the front sheet. 